194 TUE ACEPHALA. *§> 178. 



As the points of the insertion of these muscles do not always correspond 

 with the two valvos/-' their direction is often oblique, or, they sonietiniea 

 even cioss each other.''* Orhicula and Lingula, which want both the liga- 

 ment aiul hinge, have this disposition of the adductors which terminate 

 usually by a delicate tcmlon, and the contraction of either of these muscles 

 alone, produces the lateral movements of their valves. 



The spirally-pointed tentacles of the Brachiopoda are moved by a par- 

 ti^nilar apparatus. The fringes of these organs are inserted upon a carti- 

 laginous tubular jirolongation which tapers to a point. This is closed at 

 both extremities and contains a liquid, which, by the contractions of the 

 circular muscular hbres, is propelled from the base to the extremity, thereby 

 unrolling the spiral turns.*" These tentacles here certainly take the place 

 of the elastic ligament of the bivalves, for their extension probably tends 

 to slightly open the valves. 



With the Lamellibranchia, the tentacles which are not rolled, are ar- 

 ranged quite differently. Like their branchiae, they are scarcely at all 

 irritable, — this being due to the fewness of their muscular fibres. But in 

 the mantle these fibres are very abundant, and especially near their free 

 borders. This is ti;ue also of the Siphon, in which both longitudinal and 

 circular fibres can be easily seen ; here, two very distinct flattened muscles 

 arise from the base of the siphon and are inserted upon the two valves, ex- 

 ternal to the posterior adductor muscle : these serve as a Retractor siphonis-, 



§ 178. 



Very many of the Lamellibranchia have a highly-developed organ of 

 locomotion, — the /oof. ^^' This is a muscular prolongation from the ventral 

 surface, which passes obliquely forward to be inserted upon the internal sur- 

 face of the back of the shell, by four, rarely more, tendinous cords.*-' 



These cords surround the abdominal viscera, and becoming gradually 

 thicker and more muscular, finally blend with numerous, inteidaced muscu- 

 lar fasciculi which compose the foot. This last varies considerably as to its 

 size and form, and can be protruded a long way out through the open shells, 

 but may also be wholly withdrawn.'''' 



2 Lingula forms an exception. Here the prin- visceral sac to the valves. With Lingula, there 



Cipal adductor is a short, solid muscle, which are, beside the principal adductor, four pairs of in- 



Btretches straight across from one shell to the terlaced muscles, which pass obliquely tin-ough the 



other, at tlieir posterior extremity. centre of the cavity of the valves, and are attached 



■" This muscular ajjparatus has teen particularly by their two ends to the visceral sac. 



descrilx-d and figured by Owen, and f''ogt (loc. -4 Owen, Ix. cit. ; and Vogt, Anat. d. Lingula, 



cit.) with Te.rebratula, Orliicula, and Lingula. p. 8, Tab. II. fig. 16-18. 



With the Terel/ratiila, two jjairs of inusdes 1 The foot is absent, particularly with those mol- 



arise from each valve. The two anterior ones Insks which are fixed to rocks and other solid 



arising from the imperforate valve, are the longer, l>odies, by a calcareous cement, 



and liave their origin back of its centre. After the 2 Usually one pair of these delicate cords passes 



crossing of their delicate tendons, they pass above, and another below, and are inserted upon 



tlirough the opening of the valve and terminate in the valves, quite near the four points of insertion 



the peduncle, together with the two posterior ones of tlie two adductor nniscles. This is so with A7io- 



which are sliort and fleshy, and which arise at the dorita, Unio, Cardium, &c. With Isocardia^ I 



baso of the huige. Of thosf of the jjerforateU have found a thu'd pair of cords inserted upon the 



valve, the two posterior ones only pass to the posterior extremity of the simnnits of the shell, 



peduncle — tlie two anterior being attached to the These serve not only as Retractores of the foot, 



base of the other valve. With Orbicula, there but wh"n this last is fixed to some point, draw the 



are two posterior, and two anterior fleshy muscles, animal towards it. 



all of which pass obhquely from one valve to S The laterally-compressed foot of ^inodonia, ami 



the other, while the anterior ones sometimes send Unio, arises, by a large base, from the abdomen, 



off fibres to the short jieduncle. In the space cir- and has carinated borders. That of Pec2uncH/u.y 



cumscribed by Uiese four nmscles, lie four others, and f-'enus, is quite similar, but its free border is 



which are small and interlaced, and extend from tlie hollowed by a fui'row, and is therefore bi-carinated. 



